Disrupted Ultradian Activity Rhythms and Differential Expression of Several Clock Genes in Interleukin-6-Deficient Mice

Abstract

The characteristics of the cycles of activity and rest stand out among the most intensively investigated aspects of circadian rhythmicity in humans and experimental animals. Alterations in the circadian patterns of activity and rest are strongly linked to cognitive and emotional dysfunctions in severe mental illnesses such as Alzheimer's disease (AD) and major depression (MDD). The proinflammatory cytokine interleukin 6 (IL-6) has been prominently associated with the pathogenesis of AD and MDD. However, the potential involvement of IL-6 in the modulation of the diurnal rhythms of activity and rest has not been investigated. Here, we set out to study the role of IL-6 in circadian rhythmicity through the characterization of patterns of behavioral locomotor activity in IL-6 knockout (IL-6 KO) mice and wild-type littermate controls. Deletion of IL-6 did not alter the length of the circadian period or the amount of locomotor activity under either light-entrained or free-running conditions. IL-6 KO mice also presented a normal phase shift in response to light exposure at night. However, the temporal architecture of the behavioral rhythmicity throughout the day, as characterized by the quantity of ultradian activity bouts, was significantly impaired under light-entrained and free-running conditions in IL-6 KO. Moreover, the assessment of clock gene expression in the hippocampus, a brain region involved in AD and depression, revealed altered levels of cry1, dec2, and rev-erb-beta in IL-6 KO mice. These data propose that IL-6 participates in the regulation of ultradian activity/rest rhythmicity and clock gene expression in the mammalian brain. Furthermore, we propose IL-6-dependent circadian misalignment as a common pathogenetic principle in some neurodegenerative and neuropsychiatric disorders.

Keywords: behavior; circadian activity; clock gene; hippocampus; interleukin 6; mouse.

Figure 1

Experimental paradigm for the evaluation of light-entrained and free-running circadian rhythms in interleukin (IL)-6 knockout (IL-6 KO) and wild-type (WT) mice. Illustration of the temporal course (in days) for the analysis of circadian behavioral locomotor activity in IL-6 KO and WT mice under light-entrainment [light/dark (LD): 12 h light and 12 h dark phase; white boxes] and during settings of free-running rhythms [dark/dark (DD): 24 h constant darkness, black boxes].

Figure 2

Entrainment period (T), wheel-running activity, and bout analysis in under light-entrained [light/dark] conditions in interleukin (IL)-6 knockout (IL-6 KO) and wild-type (WT) mice. Analysis of the light-entrained circadian behavioral locomotor activity in IL-6 KO and WT mice (n = 9–11 per group) demonstrating comparable (A) T and wheel-running activity during the (B) alpha and (C) rho phase and in (D) total amounts. (E) Significantly increased quantity of activity bouts in IL-6 KO compared with WT mice with unaltered (F) bout length and (G) activity counts/bout. All data are displayed as mean ± SEM; *p < 0.05.

Figure 3

Circadian period (tau), wheel-running activity, bout analysis, and phase shift response under free-running (dark/dark) conditions in interleukin (IL)-6 knockout (IL-6 KO) and wild-type (WT) mice. Analysis of the free-running circadian behavioral locomotor activity in IL-6 KO and WT mice (n = 9–11 per group) demonstrating comparable (A) tau and wheel-running activity during the (B) alpha and (C) rho phase and in (D) total amounts. (E) Significantly increased quantity of activity bouts in IL-6 KO compared with WT mice with unaltered (F) bout length and (G) activity counts/bout. (H) Unaltered phase shift response to a brief light pulse at CT14 is in IL-6 KO mice. All data are displayed as mean ± SEM; *p < 0.05.

Figure 4

Behavioral actograms exemplifying circadian locomotor activity patterns in interleukin (IL)-6 knockout (IL-6 KO) and wild-type (WT) mice. Sample actograms illustrating wheel-running activity in (A) WT and (B) IL-6 KO mice.

Figure 5

mRNA levels of clock genes with significantly different expression in hippocampal tissue of interleukin (IL)-6 knockout (IL-6 KO) compared with wild-type (WT) mice. Relative expression of (A) cry1, (B) dec2, and (C) rev-erb beta in hippocampal tissue of IL-6 KO compared with WT mice (n = 6–9 per group). All data are data displayed as mean ± SEM. *p < 0.05, **p < 0.01.